The invention relates to an analog power amplifier (PA), particularly for a closed loop signal amplification requiring high output power, high efficiency, extremely high speed and wide operating temperature range.
Power amplifiers are devices designed to amplify an input signal and provide an undistorted high power output signal, i.e. an alternate current within a wide range of an output voltage; independent of supply voltages, load fluctuations over frequency, operating temperature, etc. The parameters such as input impedance, input offset voltage or open loop gain are not essential.
Conventional PAs comprise single-ended stages which results in a low and asymmetrical slew rate and a slow recovery from clipping. Multiple stages cause cumulative phase shift which impairs the stability. Crossover distortions are minimized by excessive quiescent current causing an extensive power dissipation even with no load. The switching of the power transistors is not completely eliminated however, as each transistor is cut off at a higher output current of the complementary transistor, even worse, a reverse base-emitter voltage is applied thereto.
The output stage with power transistors having emitters or sources coupled is mandatory. This results in a very slow signal amplification, wherein the voltage gain is below one. For instance, the parameter f.sub.T defining the speed of a transistor, being already very poor for power transistors and representing the bandwidth of the PA, is further reduced to a fraction. This also applies to so called transconductance amplifiers which employ a common collector rather common emitter configuration of the power transistors. Low open loop output impedance causes an increased transient intermodulation distortion (TIM) as a too late arriving feedback in response to a fast changing input signal of the PA results in a stiff, uncontrollable load driving.
A thermal compensation, simple in principle, is very inaccurate, unreliable and difficult to accomplish; it demands temperature compensation of at least two complementary high power transistors by means of floating low power components thermally coupled thereto. The quiescent current must be adjusted manually, whereby its stability is very poor. A matching of power transistor is laborious and troublesome, and in case of MOSFETs very difficult to achieve. Furthermore, the temperature coefficient of the MOSFET's gate-source voltage varies in a very wide range and has a zero value at a drain current mostly much higher than a desired quiescent current. Collectors or drains of the transistors, coupled to the cases thereof during a manufacturing process, are on different potentials. Insulating wafers introduce large parasitic capacitances and increase thermal impedance keeping individual devices at more uneven temperature.
The output voltage swing is reduced by the base-emitter or gate-source voltages of the power transistors which can be significant for high output currents and is generally very high for power MOSFETs. In order to improve the stability and accuracy of the quiescent current and minimize the likehood of thermal runaway of the power transistors, power resistors coupled is series with the load are used.
The efficiency is improved by providing a multiple level power supply. The supply voltage of the PA is switched to different values according to the output signal level, usually by means of power switches. The transition must occur at a relatively high voltage across a corresponding power transistor of the amplifier in order to prevent its saturation. The instant voltage switching at a high output current puts an enormous stress on the power transistors designed for a signal amplification, and degrades their long-term reliability.
The invention is intended to solve the above problems and therefore the object of the invention is to provide a PA having high output power, high efficiency, extremely high speed, wide operating temperature range and a very low number of components. According to the invention a PA with push-pull stages, power transistors coupled for fastest possible signal amplification and a bias circuit completely eliminating switching of the power transistors solves these problems. Moreover, a very high efficiency can be easily obtained thru an employment of multiple PAs, one of which can be greatly simplified, and a nonlinear amplifier which completely eliminates controllable power switches.
A PA according to the present invention includes an input amplifier means for amplifying the input signal of the PA and providing a first and second output signals, a first and second transistors of a first and second conductivity types respectively each having a base, emitter and collector, wherein the emitters are coupled together and the base of the first transistor is coupled to receive the first output signal of the input amplifier means, a third transistor of the first conductivity type having a first and second electrodes and also having a collector coupled to the collector of the second transistor for providing an output signal of the PA, a means for applying the second output signal of the input amplifier means to the first electrode of the third transistor, a first, second and third voltage sources, a first means for coupling the first voltage source to the collector of the first transistor, a second means for coupling the second voltage source to the base of the second transistor, and a third means for coupling the third voltage source to the second electrode of the third transistor.
In another embodiment PA includes a nonlinear amplifier means for amplifying an input signal of the PA and providing a plurality of interim signals, and power amplifier means for separately amplifying each of the interim signals and for combining the amplified interim signals into a single output signal of the PA.